Detection of corrosion using dispersed embedded sensors

ABSTRACT

A corrosion sensor system includes one or more corrosion sensors embedded in a coating material such as an anti-corrosion coating material. Each corrosion sensor may include a resonator disposed on a dielectric substrate, and has a resonant frequency in a radio frequency (RF) range or an infrared (IR) range, and is configured for interacting with an RF or IR excitation signal to produce an RF or IR measurement signal. The corrosion sensor system may be applied to an object for which corrosion is to be monitored. A corrosion detection system includes a data acquisition system that transmits the excitation signal to the corrosion sensor, and receives the measurement signal from the corrosion sensor for analysis to determine whether corrosion has occurred.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/199,584, filed Jul. 31, 2015, titled “DETECTION OFCORROSION USING DISPERSED EMBEDDED SENSORS,” the content of which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates generally to detection of corrosion, andmore particularly to detection of corrosion utilizing sensors embeddedin a material such a coating applied to an object susceptible tocorrosion.

BACKGROUND

Corrosion generally refers to an electrochemical process by which ametal is converted to metal oxides. Corrosion is destructive to metallicarticles of manufacture and therefore is undesirable. To prevent or slowdown the process of corrosion, an anti-corrosion (corrosion-resistant)coating may be applied to the outer surface of a metal, i.e., thesurface exposed to the environment. Generally an anti-corrosion coatingis, or includes, a formulation effective for resisting degradation dueto moisture, salt, oxidation, or chemical exposure. Anti-corrosivebehavior provides additional protection in altering the fundamentalmechanisms for corrosion, in addition to slowing them down. The adequatefunction of such coatings is critical, as premature failure of valuablearticles of manufacture due to corrosion can have significant financialconsequences as well as pose safety risks in the case of objectsproviding structural support. Therefore, for many types of metalobjects, it is desirable to monitor corrosive activity to facilitate theability to take remedial action as needed and on a timely basis.Anti-corrosion coatings, however, are not inherently capable ofmonitoring corrosive activity.

Therefore, it would be desirable to add functionality to anti-corrosioncoatings and other types of coatings that enables the monitoring andearly detection of corrosion. Such functionality could provide asignificant value-added feature that could save time and money for theend user and, depending on the type of metal article involved, promotesafety measures such as by enhancing the prevention of catastrophicstructural failure.

SUMMARY

To address the foregoing problems, in whole or in part, and/or otherproblems that may have been observed by persons skilled in the art, thepresent disclosure provides methods, processes, systems, apparatus,instruments, and/or devices, as described by way of example inimplementations set forth below.

According to one embodiment, a corrosion sensor system includes: acoating material; and a corrosion sensor embedded in the coatingmaterial, the corrosion sensor comprising a dielectric substrate and aresonator disposed on the dielectric substrate, wherein: the coatingmaterial, the dielectric substrate, and the resonator define a resonantfrequency of the corrosion sensor in a radio frequency (RF) range or aninfrared (IR) range; and the resonator is configured for interactingwith an RF or IR excitation signal to produce an RF or IR measurementsignal.

According to another embodiment, the corrosion sensor system includes aplurality of corrosion sensors dispersed throughout the coatingmaterial.

According to another embodiment, a corrosion detection system includes:one or more corrosion sensors; and a data acquisition system comprisingan RF or IR excitation signal source, a transceiver configured fortransmitting an excitation signal to one or more of the corrosionsensors and for receiving a measurement signal from one or more of thecorrosion sensors, and an RF or IR signal analyzer configured foranalyzing the measurement signal.

In some embodiments, the corrosion sensor has a baseline resonantfrequency corresponding to an absence of corrosion in a sensing regionproximate to and surrounding the corrosion sensor, and the signalanalyzer is configured for: measuring a wavelength or frequency spectrumof the measurement signal; finding a peak or notch wavelength orfrequency of the measured spectrum; based on the peak or notchwavelength or frequency, calculating an actual resonant frequency of thecorrosion sensor; and determining whether corrosion has occurred in thesensing region based on a difference between the baseline resonantfrequency and the actual resonant frequency.

In some embodiments, the wavelength or frequency spectrum comprises RFsignal magnitude as a function of wavelength or frequency, IRreflectance as a function of wavelength, or IR emission as a function ofwavelength.

According to another embodiment, a method for detecting corrosion of anobject includes: transmitting an RF or IR excitation signal whosewavelength or frequency vary in a predetermined range to a corrosionsensor embedded in a coating material disposed on the object, whereinthe corrosion sensor has a baseline resonant frequency corresponding toan absence of corrosion in a sensing region proximate to and surroundingthe corrosion sensor, and detecting any change in the measured responseof the corrosion sensor to receiving the RF or IR excitation signal;measuring a wavelength-dependent parameter of the measurement signal;based on the measured parameter, calculating an actual resonantfrequency of the corrosion sensor; and determining whether corrosion hasoccurred in the sensing region based on a difference between thebaseline resonant frequency and the actual resonant frequency.

According to another embodiment, a corrosion detection system isconfigured for performing any of the methods disclosed herein.

Other devices, apparatus, systems, methods, features and advantages ofthe invention will be or will become apparent to one with skill in theart upon examination of the following figures and detailed description.It is intended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood by referring to the followingfigures. The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. In the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a schematic view of an example of a corrosion detection systemaccording to some embodiments.

FIG. 2A is a schematic side view of an example of a wafer-levelstructure that may be fabricated as part of fabricating corrosionsensors according to some embodiments.

FIG. 2B is a schematic side view of an example of multiple die cut fromthe wafer-level structure illustrated in FIG. 2A.

FIG. 3A is a schematic top view of an example of a radio frequency (RF)corrosion sensor according to one embodiment.

FIG. 3B is a schematic top view of an RF corrosion sensor according toanother embodiment.

FIG. 4 is a graph of an example of a spectral response of an RFcorrosion sensor as may be acquired from an RF measurement signal.

FIG. 5A is a schematic side view of an example of another wafer-levelstructure that may be fabricated as part of fabricating corrosionsensors according to some embodiments.

FIG. 5B is a schematic side view of the wafer-level structureillustrated in FIG. 5A after etching.

FIG. 5C is a schematic side view of an example of multiple infrared (IR)corrosion sensors released from the wafer-level structure illustrated inFIGS. 5A and 5B.

FIG. 6A is a schematic plan view of an example of a frequency-selectivesurface (FSS) that may be included in an IR corrosion sensor accordingto some embodiments.

FIG. 6B is a schematic plan view of another example of an FSS that maybe included in an IR corrosion sensor according to some embodiments.

FIG. 7A is a graph of an example of a spectral response of an IRcorrosion sensor according to one mode of operation, as may be acquiredfrom an IR measurement signal.

FIG. 7B is a graph of an example of a spectral response of an IRcorrosion sensor according to another mode of operation.

FIG. 8 is a schematic view of an example of a data acquisition systemconfigured for IR operation according to some embodiments.

DETAILED DESCRIPTION

According to an aspect of the present disclosure, a corrosion sensingsystem is provided. The corrosion sensing system is formed by embeddingone or more corrosion sensors in a coating material (or, more simply, a“coating”). The corrosion sensing system may be applied as a coating tothe surface of any object for which corrosion sensing is desired. Asdescribed further below by way of non-limiting examples, the corrosionsensors are electromagnetic devices operating in, or sensitive towavelengths/frequencies in, the radio frequency (RF) or infrared (IR)spectrum. The corrosion sensors are configured for interacting withreceived electromagnetic (RF or IR) excitation signals to produceelectromagnetic (RF or IR) measurement signals. The corrosion sensorsmay be passive devices, powered only by incident excitation signals. Themeasurement signal or signals produced by a corrosion sensor may beanalyzed to determine whether corrosion has occurred (or is occurring)in the vicinity (sensing region) of that corrosion sensor.

The corrosion sensing system may be considered as a composite orsuspension of one or more corrosion sensors and a coating. The coatingserves as a matrix supporting the corrosion sensor, or dispersion ofmultiple corrosion sensors, in proximity to the object for whichcorrosion is being monitored. The coating surrounding each corrosionsensor also influences the resonant frequency of that corrosion sensor,thereby enabling corrosion sensing as described further below. Thecoating may serve other, more conventional functions as described below.Thus, embodiments disclosed herein add a functionality to coatings thatenables the monitoring and early detection of corrosion. Thiscorrosion-sensing functionality is integral to the coating itself ratherthan, for example, being a discrete patch that is part of the coatedobject. The corrosion sensors are chemically inert structures and thustheir addition to the coating does not appreciably modify the coating.Moreover, the addition of the corrosion sensors does not appreciablymodify the process for coating the object. As such, thecorrosion-sensing functionality resulting from adding the corrosionsensors involves minimal interaction with the coating itself, and thusavoids unintended consequences that might result from modifying thecoating or the coating process. Moreover, as the corrosion sensorsreceive excitation signals and transmit measurement signals wirelessly,the corrosion-sensing functionality imparted to the coating ismeasurable noninvasively, i.e., without contact with or damage to thecoating or the object. In addition, the interrogation process (i.e.,transmitting excitation signals to and measuring response signals fromthe corrosion sensors) may be done without specialized training.Likewise, the data encoded in the measurement signals may be easilyinterpreted without specialized training, such as by providing anappropriate data acquisition system according to some embodimentsdisclosed herein. The corrosion-sensing functionality may besufficiently sensitive to detect corrosion prior to excessive damage, orin some embodiments even before the onset of corrosion.

In typical embodiments, multiple corrosion sensors are dispersedthroughout the coating. The number and density of corrosion sensorsincluded in a given volume of coating may vary from one application toanother. The number and density of corrosion sensors may depend in parton factors such as, for example, the size of the surface area of theobject covered by the coating (i.e., the area over which corrosionsensing is desired), and the size of the effective sensing regionsurrounding each corrosion sensor (i.e., the sensing limit of thecorrosion sensor, or the spatial range in which corrosion sensing by thecorrosion sensor is effective). In some embodiments, the size of thecorrosion sensor is on the scale of micrometers (μm). Thus in a givenapplication, a corrosion sensing system as disclosed herein, after beingapplied to an object, may potentially include thousands or millions ofindividual corrosion sensors distributed throughout the coating. Thesize of the corrosion sensor may be defined by the maximum spatialdimension of the corrosion sensor, which is in turn defined by itsshape. The corrosion sensor may, for example, have a polygonal(prismatic) shape (e.g., a chip), in which case the maximum spatialdimension may be the length of a side of the corrosion sensor.Alternatively, the corrosion sensor may be shaped as a disk, in whichcase the maximum spatial dimension may be the diameter of the disk. Asexamples, the maximum spatial dimension of the corrosion sensor may be1000 μm or less (e.g., 1 μm to 1000 μm), or 500 μm or less, or 100 μm orless, or 50 μm or less. As additional examples, the maximum spatialdimension of the corrosion sensor may be in a range from 5 μm to 50 μm,or from 5 μm to 20 μm, or from 5 μm to 10 μm.

In the context of the present disclosure, a “coating” or “coatingmaterial” generally refers to any material that may be applied to thesurface of an object so as to cover or “coat” that surface in apermanent manner and thereby form a barrier between the surface and theenvironment. As examples, a coating may be a layer or film of materialdisposed on a surface. Generally, no limitations are placed on thecomposition, thickness, properties, function or intended use of thecoating. The coating may be transparent, translucent, or opaque. In someembodiments, the coating is an anti-corrosion coating. Examples ofanti-corrosion coatings include, but are not limited to, epoxy,polyurethane, strontium chromate, and zinc phosphate. Alternatively, thecoating may be any other type of coating applied to a surface. As oneexample, the coating may be a paint or ink intended to impart a color tothe object, i.e., a coating containing a colorant such as a pigment ordye. Alternatively or additionally, the coating may be formulated toimpart other types of optical properties to a surface, such as toenhance or suppress reflectivity or gloss level.

Generally, the corrosion sensing system may be fabricated by any processthat involves adding one or more corrosion sensors to a quantity ofcoating material. In some embodiments, the coating is fabricated in amanner conventional for its type. For example, an anti-corrosion coatingmay be fabricated using a known process for fabricating anti-corrosioncoatings. In some embodiments, the coating is provided in aprefabricated form, i.e., as a commercially available or commercialoff-the-shelf (COTS) product. After the coating has been provided(fabricated on-site or acquired off-the-shelf), the corrosion sensorsmay be added generally in any fashion that does not damage or alter thecorrosion sensors, such as by operating an appropriate dispensingapparatus. After so forming the corrosion sensing system, the corrosionsensing system may be applied as a coating (e.g., in the manner of aconventional coating) to any object for which corrosion sensing isdesired. The corrosion sensing system may be applied to the object inany manner, for example, spraying, dipping, spinning on, printing,painting, etc. The corrosion sensing system may initially be provided ina liquid or other flowable form, with the corrosion sensors suspended inthe matrix of the coating material. The manner in which the coatingmaterial adheres or bonds to the surface in a permanent manner maygenerally depend on the type of coating material. After being applied tothe object, the mechanism by which the coating material sets into apermanent, solid form on the object (e.g., drying, curing,cross-linking, etc.) may likewise generally depend on the on the type ofcoating material. Prior to applying the corrosion sensing system, thesystem may if needed or desired be physically perturbed (e.g., tumbled,agitated, etc.) to promote or enhance the distribution of the corrosionsensors throughout the bulk of the coating material.

FIG. 1 is a schematic view of an example of a corrosion detection system100 according to some embodiments. The corrosion detection system 100may generally include a corrosion sensing system 110 and a dataacquisition system 120. As described above, the corrosion sensing system110 includes a coating material 112 and a plurality of corrosion sensors114 dispersed in the coating material 112, i.e., embedded in the coatingmaterial 112 in a distributed manner. FIG. 1 illustrates the corrosionsensing system 110 after it has been applied to a surface 104 of anobject 108 to be monitored for corrosion. In other embodiments,particularly very small-scale applications, the corrosion sensing system110 may include just a single corrosion sensor 114 or a few corrosionsensors 114. In operation, the data acquisition system 120 communicateswith the corrosion sensors 114 via RF or IR excitation signals 122 andRF or IR measurement signals 124, as described further below.

In some embodiments, the configuration and operation of the corrosionsensors 114 are based on the observation that corrosive activity at theobject surface 104 leads to compositional changes in the coatingmaterial 112, which in turn alters one or more electrical properties ofthe coating material 112 such as conductivity, dielectric properties,etc. In such embodiments, the corrosion sensors 114 each are configuredto output a measurement signal that may be received by the dataacquisition system 120 and decoded so as to determine whether a changein an electrical property is indicative of the occurrence of corrosionwithin the sensing region of the corrosion sensor 114. For this purpose,the corrosion sensor 114 may be modeled as a distributed-element orlumped-element RLC circuit (with a characteristic resistance,inductance, and capacitance) having a resonant frequency and responsiveto RF wavelengths. Such a corrosion sensor 114, when initiallyintegrated into the coating material 112 and when the resultingcorrosion sensing system 110 is initially applied to the object 108, hasan initial or baseline resonant frequency and quality factor (Q factor,or simply Q) corresponding to the absence of corrosion in the sensingregion. In the present context, the quality factor Q may be defined asratio of the resonant frequency (f_(r)) to the bandwidth (Δf) of theresonator of the corrosion sensor 114. When the corrosion sensor 114 isintegrated into the coating material 112, the corrosion sensor 114 ineffect integrates the surrounding volume (sensing region) of the coatingmaterial 112 as a “parasitic” component of the RLC circuit. Hence, theresonant frequency and the Q of the RLC circuit depend at least in partupon the electrical properties of the coating material 112, such that asubsequent change in one or more electrical properties of the coatingmaterial 112 due to corrosive activity will result in a change, orshift, in the resonant frequency and/or the Q of the RLC circuit. Anindication of a change in resonant frequency (i.e., one or moremeasurable electrical properties) is encoded in the measurement signaloutputted by the corrosion sensor 114. The data acquisition system 120may be configured to analyze the signature of the measurement signal ina manner that determines whether the resonant frequency and/or the Q ofthe corrosion sensor 114 has shifted, and whether such shift isindicative of corrosion. The data acquisition system 120 may beconfigured to detect and quantify a shift in resonant frequency and/orthe Q by, for example, comparing the measured or calculated resonantfrequency and/or the Q with the known baseline resonant frequency and/orthe Q of the corrosion sensor 114. As the coating material 112surrounding the corrosion sensor 114 affects the electrical propertiesof the corrosion sensor 114 such as resonant frequency and Q factor,such electrical properties may be considered as being associated withthe overall corrosion sensing system 110 (i.e., the corrosion sensor 114embedded in the coating material 112).

In some embodiments, the corrosion sensor 114 includes a dielectricsubstrate and a resonator disposed on the dielectric substrate. Theresonator has a resonant frequency in the RF range, and is configuredfor interacting with an RF excitation signal. The resonant frequency ofthe corrosion sensor 114 may be dictated in part by the dimensions ofthe features of the resonator. As noted above, in some embodiments thecorrosion sensor 114 may have micrometer-scale dimensions. Such acorrosion sensor 114 may be fabricated according to techniques known inthe fields of microelectronics and/or microelectromechanical systems(MEMS). The corrosion sensor 114 may be formed as a planar chip (orchiplet) with a thickness defined between opposing planar sides. Thecross-section of the planar sides may be polygonal or rounded. Theresonator and the dielectric substrate may be arranged as (or similarto) an RF transmission line or as discrete L/C components. For example,the corrosion sensor 114 may have a planar geometry similar to amicrostrip or stripline transmission line. The resonator, the dielectricsubstrate, and the surrounding coating material may together define theresonant frequency, Q factor, and other electrical properties of thecorrosion sensor 114.

FIGS. 2A to 4 illustrate non-limiting examples of an RF corrosion sensoraccording to some embodiments.

FIGS. 2A and 2B illustrate an example of fabricating RF corrosionsensors 214 by implementing a wafer-level microelectronic fabricationprocess. Specifically, FIG. 2A is a schematic side view of an example ofa wafer-level structure 206, and FIG. 2B is a schematic side view of anexample of multiple die cut from the wafer-level structure 206.Referring to FIG. 2A, a wafer-sized dielectric substrate 232 isprovided. Generally, the dielectric substrate 232 may be composed of anydielectric material suitable for high-frequency (RF) transmission. Ametallization layer is deposited on a first side (top side, from theperspective of FIG. 2A) of the dielectric substrate 232. Themetallization layer is then patterned (e.g., by lithography) to formresonators 234, each resonator 234 being defined by a pattern of one ormore metal traces on the dielectric substrate 232. Another metallizationlayer can be deposited on an opposing second side (bottom side, from theperspective of FIG. 2A) of the dielectric substrate 232 to form anoptional ground plane 236. An appropriate dicing technique (e.g., wafersawing) is then performed to separate die from the wafer-level structure206. As shown in FIG. 2B, each die corresponds to an individual RFcorrosion sensor 214. The RF corrosion sensors 214 may then beintegrated with a coating to form a corrosion sensing system (e.g., thecorrosion sensing system 110 shown in FIG. 1) as described above.

FIG. 3A is a schematic top view of an example of an RF corrosion sensor314A according to one embodiment. FIG. 3A illustrates one non-limitingexample of a resonator 334A formed by a metal trace pattern on adielectric substrate 332A. Two ends of the metal strip are separated bya small gap and form a controlled capacitance 338A that influences theresonant frequency of the resonator 334A. When the RF corrosion sensor314A is integrated with a coating, the controlled capacitance 338A isperturbed by the coating and thus is sensitive to changes in the coatingdue to corrosion. At least a portion of the resonator 334A is effectiveas an RF antenna capable of receiving and interacting with RF excitationsignals to produce RF measurement signals.

FIG. 3B is a schematic top view of an example of an RF corrosion sensor314B according to another embodiment. The RF corrosion sensor 314Bincludes a resonator 334B formed by metal traces on a dielectricsubstrate 332B. Two overlapping ends of the metal traces are separatedby a small gap and form a controlled capacitance 338B. Persons skilledin the art will appreciate that many variants of the configurationsshown in FIGS. 3A and 3B, the materials utilized, and the fabricationtechniques employed are possible.

In operation, an appropriate RF transmission device (such as may be partof the data acquisition system 120 illustrated in FIG. 1 and describedfurther below) is operated to interrogate an RF corrosion sensor such asdescribed above and illustrated in FIGS. 2A to 3B by transmitting an RFexcitation signal to the RF corrosion sensor. The incident excitationsignal may be swept through a range of RF frequencies that extend aboveand below the resonant frequency of the RF corrosion sensor. As theincident RF signal is swept, at frequencies near to the resonantfrequency of the RF corrosion sensor, some of the RF energy will beabsorbed by the RF corrosion sensor. At frequencies farther away fromthe resonant frequency of the RF corrosion sensor, very little if any RFenergy is absorbed. This response of the RF corrosion sensor to theincident radiation may be detected by measuring, for example, the S11signal of the RF incident signal (a well-known scattering parameter, or“S-parameter,” commonly utilized in measurement of linear electricalnetworks). A dip (or notch, well, inverse peak) in the S11 signal can beanalyzed to determine the resonant frequency and/or the Q factor of theresonance. A change in the resonant frequency and/or the Q may beevaluated to determine whether the change is an indication that thedielectric and/or conductive properties of the coating proximate to thesensor have changed in response to corrosion occurring nearby.

FIG. 4 is a graph of an example of a spectral response of an RFcorrosion sensor as may be acquired from an RF measurement signal.Specifically, FIG. 4 is a plot of the S11 parameter (in decibels) of theRF corrosion sensor as a function of frequency (in megahertz). Thespectrum includes a dip corresponding to strong absorbance by theresonator at a certain frequency (350 MHz in the illustrated example,corresponding to the local minimum of the dip).

In some embodiments, the RF corrosion sensor may have a baselineresonant frequency in a range from 100 MHz to 200 GHz, although in otherembodiments the baseline resonant frequency may be above or below thisrange. As noted above, in certain embodiments the RF corrosion sensormay have a maximum spatial dimension of less than 50 μm. In someembodiments, an RF corrosion sensor of this scale may be configured tooperate in the region around 94 GHz. The 94 GHz region is of interestfor two primary reasons. First, the wavelength of 94 GHz radio waves isshort (3 mm), which is believed to be feasible for self-resonance in adevice scaled at less than 50 μm. Second, present commercialtechnologies using 94 GHz for automotive and other applications arebeing actively developed, which may facilitate implementation ofembodiments of RF-based corrosion sensing disclosed herein.

FIGS. 5A to 7B illustrate non-limiting examples of an IR corrosionsensor according to some embodiments.

FIGS. 5A to 5C illustrate an example of fabricating IR corrosion sensors514 in a fashion similar to the wafer-level fabrication processdescribed above. Specifically, FIG. 5A is a schematic side view of anexample of a wafer-level structure 506, FIG. 5B is a schematic side viewof the wafer-level structure 506 after etching, and FIG. 5C is aschematic side view of an example of multiple IR corrosion sensors 514released from the wafer-level structure 506. Referring to FIG. 5A, asacrificial release layer 542 is deposited on a temporary substrate 544.The release layer 542 may have any composition suitable for its purposeas a release layer. The release layer 542 may a commercially availablematerial such as, for example, REVALPHA® tape available from NittoAmericas, Inc., Teaneck, N.J., USA, or the 3M™ Wafer Support Systemavailable from 3M Company. A wafer-sized dielectric substrate 532 isthen deposited on the release layer 542. Generally, the dielectricsubstrate 532 may be composed of any dielectric material suitable forIR-wavelength transmission. In some embodiments, the dielectric materialmay be transparent to the IR wavelengths contemplated for operation. Insome embodiments an IR-reflecting material, or reflector 536, may beformed so as to be embedded in the dielectric substrate 532. Forexample, after forming a first layer of dielectric material on therelease layer 542, a metallization layer may be deposited on the firstlayer of dielectric material to form the reflector 536. A second layerof dielectric material may then be deposited on the reflector 536, suchthat the first and second dielectric layers comprise the dielectricsubstrate 532 in which the reflector 536 is embedded.

Subsequently, another metallization layer is deposited on a first side(top side, from the perspective of FIG. 5A) of the dielectric substrate532. This metallization layer is then patterned (e.g., by lithography)to form IR-responsive resonators 534, examples of which are describedbelow.

Referring to FIG. 5B, the dielectric substrate 532 is then etched downto the release layer 542 in a pattern that defines the size and shape ofthe individual IR corrosion sensors 514. Any etching techniqueappropriate for the composition and size resolution of the materials maybe employed, such as wet (chemical) etching, dry (e.g., plasma) etching.Other techniques such as laser ablation or mechanical sawing may besuitable. Referring to FIG. 5C, an appropriate release technique(thermal or laser for example) is then performed to separate individualIR corrosion sensors 514 from the wafer-level structure 506. The IRcorrosion sensors 514 may then be integrated with a coating to form acorrosion sensing system (e.g., the corrosion sensing system 110 shownin FIG. 1) as described above. The approach of releasing the IRcorrosion sensors 514 from a wafer level structure 506 can also beapplied to the fabrication of RF corrosion sensors.

In some embodiments and as also illustrated in FIGS. 5A to 5C, the IRcorrosion sensors 514 may have a double-sided configuration in whichresonators are located on two opposing sides of the IR corrosion sensors514. Specifically, each IR corrosion sensor 514 may include, in additionto the first resonator 534 on the first side of the dielectric substrate532, a second resonator 546 on the opposing second side (bottom side,from the perspective of FIGS. 5A to 5C) of the dielectric substrate 532.

The resonator 534 (or 546) of the IR corrosion sensor 514 may beconfigured as a frequency-selective surface (FSS). The double-sided IRcorrosion sensors 514 shown in FIGS. 5A to 5C may thus have a first FSSand a second FSS. The FSS may include a periodic array or pattern ofstructural features (or cells) arranged in a two-dimensional (2D) layeron the dielectric substrate 532. The structural features are sized toenable the resonator 534 to resonate at an IR wavelength. For example,each structural feature may have a characteristic dimension (e.g.,length or diameter) in a range from 0.5 to 10 μm. In some embodiments,the structure of the resonator 534 may be considered as being within theclass of materials known as metamaterials. In some embodiments, the IRcorrosion sensor 514 may have a baseline resonant wavelength in a rangefrom 1 μm to 15 μm, although in other embodiments the baseline resonantwavelength may be above or below this range.

FIG. 6A is a schematic plan view of an example of a frequency-selectivesurface (FSS) 634A that may be provided as the resonator of an IRcorrosion sensor according to some embodiments. In this example, the FSS634A includes a 2D array of metal elements or patches uniformly spacedfrom each other. FIG. 6B is a schematic plan view of another example ofa frequency-selective surface (FSS) 634B, which includes a metal grid ormesh defining 2D array of apertures uniformly spaced from each other. Inother embodiments, the 2D array and/or the periodic features need not berectilinear. For example, an FSS may be formed by a hexagonal array ofcircular patches or apertures (not shown).

In operation, an appropriate IR transmission device (such as may be partof the data acquisition system 120 illustrated in FIG. 1 and describedfurther below) is operated to interrogate an IR corrosion sensor such asdescribed above and illustrated in FIGS. 5A to 6B by transmitting an IRexcitation signal (IR beam) to the IR corrosion sensor. The resonatormodulates the incident IR excitation signal, absorbing certainwavelengths while reflecting other wavelengths.

In one mode of operation, the IR excitation signal may be a broadbandsignal spanning a range of wavelengths that includes the wavelengthcorresponding to the known baseline resonant frequency of the resonatorand also other wavelengths corresponding to frequency values to whichthe resonant frequency may be expected to have shifted due to corrosion.The modulated signal reflected back from the resonator is utilized asthe IR measurement signal. The IR measurement signal may include a notch(i.e., a minimum value) corresponding to strong absorbance by theresonator of the incident IR excitation signal at a certain wavelength.The actual resonant frequency of the resonator may be determined fromthis IR measurement signal, and compared to the known baseline resonantfrequency to determine whether corrosion has occurred in the sensingregion of the corrosion sensor being interrogated.

In another mode of operation, the IR excitation signal may be centeredon a specific wavelength that induces emission by the resonator of an IRmeasurement signal at a different wavelength. In this mode, the IRmeasurement signal may include a peak intensity at a certain wavelength,which likewise may be analyzed to determine a shift in resonantfrequency and the occurrence of corrosion.

In either mode of operation, the provision of the reflector 536 (FIGS.5A to 5C) in the corrosion sensor may be useful for increasing theintensity of the IR measurement signal and consequently the sensitivityof the measurement. Thus, the data acquisition system may receive the IRmeasurement signal directly from the resonator (via reflection oremission) or from the reflector 536.

FIG. 7A is a graph of an example of a spectral response of an IRcorrosion sensor according to the first mode of operation, as may beacquired from an IR measurement signal. Specifically, FIG. 7A is a plotof reflectance (%) of the IR corrosion sensor as a function ofwavelength. The spectrum includes a dip or notch corresponding to strongabsorbance by the resonator at a certain wavelength. FIG. 7B is a graphof an example of a spectral response of an IR corrosion sensor accordingto the second mode of operation. Specifically, FIG. 7B is a plot ofemission (in arbitrary units) as a function of wavelength. The spectrumincludes a peak emission at a certain wavelength. Spectra such as shownin FIGS. 7A and 7B may be utilized to detect for corrosion, as describedelsewhere in the present disclosure.

An IR corrosion sensor as described above may be comparatively smallerthan an RF corrosion sensor, which may be considered advantageous insome applications. For example, an IR corrosion sensor may have amaximum spatial dimension of less than 10 μm. In other applications,however, an RF corrosion sensor may be preferred due to not requiringline-of-sight communication between the RF corrosion sensor and the RFprobe or data acquisition system. In another aspect, the cost andcomplexity of the data acquisition system may influence the choicebetween using RF corrosion sensors or IR corrosion sensors in a givenapplication.

Referring back to FIG. 1, in some embodiments the data acquisitionsystem 120 includes an RF or IR excitation signal source 152, atransceiver 154 configured for transmitting RF or IR excitation signals122 to the resonator of one or more corrosion sensors 114 and forreceiving RF or IR measurement signals 124 from the resonator, and an RFor IR signal analyzer 156 configured for analyzing the measurementsignal. The data acquisition system 120 may also include a computingdevice (or system controller) 158. FIG. 1 also schematically depicts theforegoing components enclosed in a housing 162 of the data acquisitionsystem 120. It will be understood, however, that one or more componentsmay be enclosed in one or more housings separate from the othercomponents. For example, the excitation signal source 152 and thetransceiver 154 may be housed together as a separate device or probethat communicates with the signal analyzer 156 via a wireless or wiredcommunication link (such as a removable cable or tether). Such a probemay be handheld or at least portable, or may be mountable to a guide ortrack along with the probe is moved to enable automated scanning over alarge surface area of the object 108. As another example, the computingdevice 158 may be a separate device that communicates with the signalanalyzer 156 and/or other components via a wireless or wiredcommunication link.

The excitation signal source 152 may be any device suitable forgenerating RF or IR excitation signals 122 in accordance with a givenRF-based or IR-based embodiment. For example, in an RF-based embodimentthe excitation signal source 152 may include an RF power source and anRF signal generator (e.g., frequency synthesizer). In an IR-basedembodiment the excitation signal source 152 may be a broadband lightsource that emits IR radiation (e.g., various types of lamps), or anarrowband IR source such as, for example, certain types of lightemitting diodes (LEDs), laser diodes (LDs), and lasers.

The transceiver 154 may be any device suitable for transmittingexcitation signals 122 and receiving measurement signals 124 at the RFor IR wavelengths/frequencies contemplated for operation. For example,in an RF-based embodiment the transceiver 154 may be an RF antenna orcoil. In some embodiments, separate RF antennas may be utilized fortransmitting and receiving. In an IR-based embodiment the transceiver154 includes separate devices for transmitting IR excitation signals 122and for receiving IR measurement signals 124. The IR transmitter (or IRbeam output) may simply be the output of the (IR) excitation signalsource 152, or may further include optics (e.g., windows, lenses,filters, etc.) as needed or desired. The IR receiver may be anIR-sensitive photodetector.

The signal analyzer 156 may be any analytical instrument capable ofmeasuring attributes of RF or IR signals in a manner that enablesquantification of resonant frequency and/or Q and shifts in resonantfrequency and/or Q associated with the corrosion sensor 114 beinginterrogated. The signal analyzer 156 may be an RF or IR spectrumanalyzer capable of generating RF or IR spectra from RF or IRmeasurement signals 124 such as, for example, RF signal magnitude as afunction of wavelength, IR reflectance as a function of wavelength, orIR emission as a function of wavelength. In some embodiments, the signalanalyzer 156 may be a commercially available instrument. In someIR-based embodiments, the signal analyzer 156 may be a reflectancespectrometer, an IR spectrometer, a Fourier transform IR (FTIR)spectrometer, etc.

The computing device 158 may represent one or more modules (or units, orcomponents) configured for controlling, monitoring and/or timing variousfunctional aspects of the data acquisition system 120, such as thetransmission and frequency composition of excitation signals 122, theconditioning, processing, and analysis of measurement signals 124, datalogging, etc. Depending on the embodiment, all or part of the computingdevice 158 may be integrated with the signal analyzer 156. All or partof the computing device 158 may be, or be embodied in, for example, adesktop computer, laptop computer, portable computer, tablet computer,handheld computer, mobile computing device, personal digital assistant(PDA), smartphone, etc. The computing device 158 may include softwareconfigured to execute one or more algorithms that analyze the dataencoded in received excitation signals 122, for example determining peakor dip location, amplitude, and any other information that may beutilized to determine quantitative shift in spectral response andcorrelate the spectral response with corrosion. In some embodiments, thecomputing device 158 may also be configured for providing andcontrolling a user interface that provides screen displays of spectraldata and/or other data with which a user may interact. The computingdevice 158 may include one or more reading devices on or in which atangible computer-readable (machine-readable) medium may be loaded thatincludes instructions for performing all or part of any of the methodsdisclosed herein. For all such purposes, the computing device 118 mayinclude one or more types of hardware, firmware and/or software, as wellas one or more memories and databases.

FIG. 8 is a schematic view of an example of a data acquisition system820 configured for IR operation according to some embodiments. Thetransceiver includes an IR transmitter 862 and an IR receiver 864. Asnoted above, the IR transmitter 862 may be the output of an IRexcitation signal source 852, or may be one or more separate opticalcomponents that receive the IR light generated by the IR excitationsignal source 852 and direct the IR light out from the data acquisitionsystem 820 as the IR excitation signal 122. For narrowband excitation,in some embodiments the IR excitation signal source 852 may include aplurality of light sources enabling user selection of the excitationwavelength, for example a plurality of LEDs mounted on a wheel. In otherembodiments, a plurality of selectable optical filters may be provided,for example on a filter wheel. The IR receiver 864 may include one ormore photodetectors.

For purposes of the present disclosure, it will be understood that termssuch as “communicate” and “in . . . communication with” (for example, afirst component “communicates with” or “is in communication with” asecond component) are used herein to indicate a structural, functional,mechanical, electrical, signal, optical, magnetic, electromagnetic,ionic or fluidic relationship between two or more components orelements. As such, the fact that one component is said to communicatewith a second component is not intended to exclude the possibility thatadditional components may be present between, and/or operativelyassociated or engaged with, the first and second components.

It will also be understood that when a layer (or film, region,substrate, component, device, or the like) is referred to as being “on”or “over” another layer, that layer may be directly or actually on (orover) the other layer or, alternatively, intervening layers (e.g.,buffer layers, transition layers, interlayers, sacrificial layers,etch-stop layers, masks, electrodes, interconnects, contacts, or thelike) may also be present. A layer that is “directly on” another layermeans that no intervening layer is present, unless otherwise indicated.It will also be understood that when a layer is referred to as being“on” (or “over”) another layer, that layer may cover the entire surfaceof the other layer or only a portion of the other layer. It will befurther understood that terms such as “formed on” or “disposed on” arenot intended to introduce any limitations relating to particular methodsof material transport, deposition, fabrication, surface treatment, orphysical, chemical, or ionic bonding or interaction. The term“interposed” is interpreted in a similar manner.

It will be understood that various aspects or details of the inventionmay be changed without departing from the scope of the invention.Furthermore, the foregoing description is for the purpose ofillustration only, and not for the purpose of limitation—the inventionbeing defined by the claims.

What is claimed is:
 1. A corrosion sensor system, comprising: a coatingmaterial; and a corrosion sensor embedded in the coating material, thecorrosion sensor comprising a dielectric substrate and a resonatordisposed on the dielectric substrate, wherein: the coating material isdisposed as a coating on an object to be sensed by the sensor; thecoating material, the dielectric substrate, and the resonator define aresonant frequency of the corrosion sensor in a radio frequency (RF)range or an infrared (IR) range; a data acquisition system configured tocommunicate with the corrosion sensor via RF or IR signals; and theresonator is configured for interacting with an RF or IR excitationsignal to produce an RF or IR measurement signal such that a shift in anRF or IR signal parameter determined by the data acquisition system isdependent on at least one property indicative of corrosion of the objectunderlying the corrosion sensor.
 2. The corrosion sensor system of claim1, wherein the dielectric substrate and the resonator have aconfiguration selected from the group consisting of: the dielectricsubstrate and the resonator are arranged as a planar RF or IRtransmission line; the dielectric substrate and the resonator arearranged as an RLC circuit; and both of the foregoing.
 3. The corrosionsensor system of claim 1, comprising an electrical ground plane on aside of the dielectric substrate opposite to the resonator.
 4. Thecorrosion sensor system of claim 1, wherein the resonator comprises afeature selected from the group consisting of: an RF antenna; two ormore electrically conductive sections separated by a gap and forming acapacitor; and both of the foregoing.
 5. The corrosion sensor system ofclaim 1, comprising a configuration selected from the group consistingof: the resonator comprises a frequency-selective surface (FSS); theresonator comprises a frequency-selective surface (FSS), and the FSScomprises a periodic array of structural features sized such that theFSS resonates at an IR frequency; the resonator comprises afrequency-selective surface (FSS), and the FSS comprises a periodicarray of structural features sized such that the FSS resonates at an IRfrequency, wherein the structural features comprise metal elementsspaced from each other or a metal grid defining apertures spaced fromeach other; and the resonator comprises a frequency-selective surface(FSS), and further comprising an IR-reflecting material disposed on aside of the dielectric substrate opposite to the resonator or embeddedin the dielectric substrate at a distance from the resonator, whereinthe dielectric substrate comprises an IR-transmitting material.
 6. Thecorrosion sensor system of claim 1, wherein: the dielectric substratecomprises an IR-transmitting material, a first side, and a second sideopposite the first side; the resonator comprises a firstfrequency-selective surface (FSS) on the first side and a second FSS onthe second side; and the corrosion sensor further comprises anIR-reflecting material embedded in the dielectric substrate between thefirst FSS and the second FSS.
 7. The corrosion sensor system of claim 1,wherein the corrosion sensor has a maximum spatial dimension in a rangefrom 1 to 1000 μm.
 8. The corrosion sensor system of claim 1, whereinthe resonant frequency is in a range from 100 MHz to 200 GHz.
 9. Thecorrosion sensor system of claim 1, wherein the resonant wavelength isin a range from 1 μm to 15 μm.
 10. The corrosion sensor system of claim1, comprising a plurality of corrosion sensors dispersed throughout thecoating material.
 11. The corrosion sensor system of claim 1, whereinplural corrosion sensors are disposed in the coating on the object. 12.The corrosion sensor system of claim 1, wherein the coating material isselected from the group consisting of: an anti-corrosion material; apaint; an anti-corrosion material selected from the group consisting ofepoxy, polyurethane, strontium chromate, and zinc phosphate; a liquid; asolid; and a combination of two or more of the foregoing.
 13. Acorrosion detection system, comprising: the corrosion sensor system ofclaim 1; and the data acquisition system comprising an RF or IRexcitation signal source, a transceiver configured for transmitting theexcitation signal to the resonator and for receiving the measurementsignal from the resonator, and an RF or IR signal analyzer configuredfor analyzing the measurement signal.
 14. The corrosion detection systemof claim 13, wherein the excitation source and the transceiver have aconfiguration selected from the group consisting of: the excitationsource comprises an RF signal generator and the transceiver comprises anRF antenna; and the excitation source comprises an IR light source, andthe transceiver comprises an IR beam output and an IR-sensitivephotodetector.
 15. The corrosion detection system of claim 13, whereinthe corrosion sensor has a baseline wavelength-dependent parametercorresponding to an absence of corrosion in a sensing region proximateto and surrounding the corrosion sensor, and the signal analyzer isconfigured for: measuring an actual wavelength-dependent parameter ofthe measurement signal; and determining whether corrosion has occurredin the sensing region based on a difference between the baselinewavelength-dependent parameter and the actual wavelength-dependentparameter.
 16. The corrosion detection system of claim 15, wherein thewavelength-dependent parameter is the resonant frequency, a qualityfactor, an RF signal magnitude as a function of frequency, an IRreflectance as a function of wavelength, or an IR emission as a functionof wavelength.
 17. The corrosion detection system of claim 13, whereinat least a portion of the data acquisition system comprising thetransceiver is portable.
 18. The corrosion detection system of claim 13,comprising a plurality of the corrosion sensors, wherein the corrosionsensors are dispersed throughout the coating material.
 19. A dataacquisition system for detecting corrosion of an object, the dataacquisition system comprising: a radio frequency (RF) or an infrared(IR) excitation signal source; a transceiver configured for transmittingthe RF or IR excitation signal to a corrosion sensor disposed on theobject to be sensed by the corrosion sensor, and for receiving the an RFor IR measurement signal generated by the corrosion sensor in responseto the RF or IR excitation signal; and an RF or IR signal analyzerconfigured for analyzing the measurement signal in order to determine ashift in an RF or IR signal parameter dependent on at least one propertyindicative of corrosion of the object underlying the corrosion sensor.20. The data acquisition system of claim 19, wherein the excitationsource and the transceiver have a configuration selected from the groupconsisting of: the excitation source comprises an RF signal generatorand the transceiver comprises an RF antenna; and the excitation sourcecomprises an IR light source, and the transceiver comprises an IR beamoutput and an IR-sensitive photodetector.
 21. The data acquisitionsystem of claim 19, wherein the corrosion sensor has a baselinewavelength-dependent parameter corresponding to an absence of corrosionin a sensing region proximate to and surrounding the corrosion sensor,and the signal analyzer is configured for: measuring an actualwavelength-dependent parameter of the measurement signal; anddetermining whether corrosion has occurred in the sensing region basedon a difference between the baseline wavelength-dependent parameter andthe actual wavelength-dependent parameter.
 22. The data acquisitionsystem of claim 21, wherein the wavelength-dependent parameter is aresonant frequency, a quality factor, an RF signal magnitude as afunction of frequency, an IR reflectance as a function of wavelength, oran IR emission as a function of wavelength.
 23. A method for detectingcorrosion of an object, the method comprising: transmitting a radiofrequency (RF) or an infrared (IR) excitation signal to a corrosionsensor embedded in a coating material disposed on the object to besensed by the corrosion sensor, wherein the corrosion sensor has abaseline wavelength-dependent parameter corresponding to an absence ofcorrosion of the object in a sensing region proximate to and surroundingthe corrosion sensor, and the corrosion sensor interacts with the RF orIR excitation signal to produce an RF or IR measurement signal;receiving the RF or IR measurement signal; measuring an actualwavelength-dependent parameter of the measurement signal; anddetermining whether corrosion of the object has occurred in the sensingregion based on a difference between the baseline wavelength-dependentparameter and the actual wavelength-dependent parameter.
 24. The methodof claim 23, wherein the parameter is RF signal magnitude as a functionof frequency, IR reflectance as a function of wavelength, or IR emissionas a function of wavelength.
 25. The method of claim 23, wherein: thecorrosion sensor comprises a dielectric substrate and an RF resonatordisposed on the dielectric substrate; transmitting comprisestransmitting an RF excitation signal to induce an electrical current inthe resonator, wherein the RF excitation signal is modulated by the RFresonator to form an RF measurement signal; and receiving comprisesreceiving the RF measurement signal.
 26. The method of claim 23,wherein: the corrosion sensor comprises a dielectric substrate and an IRresonator disposed on the dielectric substrate; transmitting comprisestransmitting an IR excitation signal to the IR resonator, wherein the IRexcitation signal is modulated by the IR resonator to form an IRmeasurement signal; and receiving comprises receiving the IR measurementsignal reflected by the IR resonator.
 27. The method of claim 26,wherein receiving comprises receiving the IR measurement signalreflected directly from the IR resonator, or reflected from anIR-reflecting material disposed on or embedded in the dielectricsubstrate.
 28. The method of claim 26, wherein measuring the parametercomprises finding a wavelength at which maximum absorption of the IRexcitation signal by the IR resonator occurs, or finding a wavelength atwhich maximum emission of the IR measurement signal by the IR resonatoroccurs.
 29. The method of claim 23, comprising applying the coatingmaterial to the object.